Secure Broadcasting Over Fading Channels

We study a problem of broadcasting confidential messages to multiple receivers under an information-theoretic secrecy constraint. Two scenarios are considered: 1) all receivers are to obtain a common message; and 2) each receiver is to obtain an independent message. Moreover, two models are considered: parallel channels and fast-fading channels. For the case of reversely degraded parallel channels, one eavesdropper, and an arbitrary number of legitimate receivers, we determine the secrecy capacity for transmitting a common message, and the secrecy sum-capacity for transmitting independent messages. For the case of fast-fading channels, we assume that the channel state information of the legitimate receivers is known to all the terminals, while that of the eavesdropper is known only to itself. We show that, using a suitable binning strategy, a common message can be reliably and securely transmitted at a rate independent of the number of receivers. We also show that a simple opportunistic transmission strategy is optimal for the reliable and secure transmission of independent messages in the limit of large number of receivers.     

Cross Layer Design for Multiaccess Communication Over Rayleigh Fading Channels

An information theoretic queueing model is proposed in a wireless multiple access communication setup. The proposed symmetric N user model captures physical layer parameters such as the encoding rate, transmit power and Medium Access Control (MAC) layer metrics such as queue stability. Two alternative medium access strategies are considered: centralized scheduling and ALOHA. Next, a cross-layer approach is taken wherein the maximum stable throughput of the system is achieved by a joint optimization over the MAC parameters (viz., scheduling set size with scheduling and transmission probability with ALOHA) and the encoding rate. Performance comparisons with traditional layered designs are given. It is shown that in the low and high SNR regimes, layered designs are close to optimal whereas in the moderate SNR range, cross-layer designs outperform layered schemes. Exact characterizations of the ";low"; and ";high"; SNR regimes are given quantitatively. It is also shown that ALOHA with transmission probability one is optimal in the low SNR regime.     

莱斯衰落中多用户大规模MIMO系统的安全通信

研究了莱斯衰落信道中多用户集中式大规模多输入多输出(Multiple-Input Multiple-Output,MIMO)系统的安全通信问题。基站利用上行训练获取信道状态信息,之后利用最大比传输波束成形算法进行下行数据传输。基于随机矩阵理论,分别分析了合法用户和窃听者的信息速率表达式,从而得到了遍历安全速率的闭合表达式。基于获得的结果,进一步分析了极端莱斯K因子和无限发送数据功率下系统的极限安全性能。最后,通过不同条件下的实验仿真设计,验证了系统的安全性能,同时证实了所得结果的正确性。     

Target Tracking Over Fading Channels

The problem of tracking a target in a multitarget environment when the observations are received over a fading channel is considered. The optimal Bayes solution to the tracking problem in such cases involves growing memory and hence is not feasible. A particularly effective suboptimal scheme uses a probabilistic judgment at each stage of the observations to overcome this problem. This concise paper presents an evaluation of the scheme in terms of mean-square error performance when the observations are received over fading channels.     

混沌扩频通信系统在多径Rayleigh衰落信道中的性能分析

本文研究了混沌直接扩频信号在多径Rayleigh衰落信道中传输时的抗多径干扰和多址干扰的特性。对于所给出的混沌扩频通信系统,当加入多径干扰和信道噪声时,给出了理论误码率与实际误码率的比较,数值结果表明在多径衰落信道中,混沌扩频通信系统抗多径干扰的性能很好;同时对混沌直接扩频信号的奇、偶互相关函数进行了计算,结果表明混沌扩频通信系统具有良好的抗多址干扰能力。     

在频率选择性信道中的差分空频编码

本文提出了频率选择性衰变信道中采用了差分空频编码的正交频分复用(OFDM)传输方法。根据信道长度,我们将每个天线OFDM帧中的输入数据分组,同一组中各天线上的数据编码组成为一个对角信号星座,沿频域方向独立的对每组信号实施差分编码。通过分析成对错误概率,我们证明了这种码潜在能提供的分集是发射天线数,接收天线数和信道长度的乘积,比差分空时码具有更大分集增益,因而具有更好的性能,这一分析结果也为我们的仿真实验证实了。     

Achievable Rates Over Time-Varying Rayleigh Fading Channels

This paper investigates achievable rates for a wireless communication system when neither the transmitter nor the receiver has a priori knowledge of the channel state information (CSI). The dynamics of the flat fading channel are characterized by a known Doppler spectrum. Quantitative results are provided for independent and identically distributed (i.i.d.) Gaussian signals and long data blocks. Expressions for the achievable rates include a lower bound on mutual information, and the achievable rates of pilot-aided systems with optimized resource allocation. A simple, low-duty-cycle signaling scheme is proposed to improve the information rates in the low signal-to-noise ratio (SNR) regime, and the optimal duty cycle is expressed as a function of the fading rate and SNR. It is demonstrated that the resource allocation and duty cycle developed for Gaussian signals can also be applied to systems using other signaling formats.     

Two-Dimensional DPCM Image Transmission Over Fading Channels

A combined source-channel coding approach is described for the encoding, transmission, and remote reconstruction of image data. The transmission medium considered is that of a fading dispersive communications channel. Both the Rician fading and Rayleigh fading channel models are considered. The image source encoder employs two-dimensional (2-D) differential pulse code modulation (DPCM). This is a relatively efficient encoding scheme in the absence of channel errors. In the presence of fading, however, the performance degrades rapidly. By providing error control protection to those encoded bits which contribute most significantly to image reconstruction, it is possible to minimize this degradation without sacrificing transmission bandwidth. Several modulation techniques are employed in evaluation of system performance including noncoherent multiple frequency shift-keyed (MFSK) modulation. Analytical results are provided for assumed 2-D autoregressive image models, while simulation results are described for real-world images.     

Outage Analysis of Mixed Dual-Hop RF-FSO Communication System Over Fading Channels with Pointing Errors

Wireless Personal Communications - In this paper, we consider a mixed dual-hop RF-FSO communication system, where RF link is subjected to Rice fading, while FSO link experience double generalized...     

Packet Radio Performance Over Slow Rayleigh Fading Channels

Expressions for the throughput and average packet delay for a Pure-ALOHA single-hop packet radio system operating in slow Rayleigh fading are derived. For noncoherent frequency-shift-keying (NCFSK), an exact closed form expression is presented. For coherent phase-shift-keying (CPSK) an excellent approximation for large packet sizes is derived. This approximation technique is valid in general for other modulation schemes and for other fading channel statistical characterizations. The packet length which maximizes the useful data throughput in slow Rayleigh fading is found. The results of this investigation indicate that a packet radio system can be designed with a modest link margin for fading and achieve identical throughput performance over a nonfading channel and a fading channel with only a small increase in average packet delay for the fading channel.     

Block-Differential Modulation Over Doubly Selective Wireless Fading Channels

Block-Differential Modulation Over Doubly Selective Wireless Fading Channels Differential encoding is known to simplify receiver implementation because it bypasses channel estimation. However, over rapidly fading wireless channels, extra transceiver modules are necessary to enable differential transmission. Relying on a basis-expansion model for time- and frequency-selective (doubly selective) channels, we derive such a generalized block-differential (BD) codex and prove that it achieves maximum Doppler and multipath diversity gains, while affording low-complexity maximum-likelihood decoding. We further show that existing BD systems over frequency-selective or time-selective channels follow as special cases of our novel system. Simulations using the widely accepted Jakes model corroborate our theoretical analysis.     

Multi-Antenna AF Two-Way Relaying Over Nakagami-m Fading Channels

We present the performance of multi-antenna selective combining amplify-and-forward (SC-AF) two-way relaying systems over independent and identically distributed (i.i.d) Nakagami- $m$ fading channels. The outage probability and symbol error probability of our relaying system are derived in closed-form. In order to get additional insights into the impact of system parameters, we consider the analysis of system at high signal-to-noise (SNR) regime. Through over high SNR analysis, we assume that the fading channels are independent and non-identically distributed (i.n.i.d). Also, there is no assumption on the fading parameter, $m$ . Moreover, some special cases of practical interest (e.g., Rayleigh fading channels, and single relay system) are also examined. Subsequently, we define an optimization problem using some approximations and then solve it analytically. Lastly, we present numerical simulations to check our analytical formulas.     

On Fading Margin in Ultrawideband Communications Over Multipath Channels

This work studies ultra wideband (UWB) communications over multipath residential indoor channels. We study the relationship between the fading margin and the transmitter–receiver separation distance for both the line of sight and the no line of sight scenarios. Impairments such as small scale fading as well as large scale fading are considered. Some implications of the results for UWB indoor network design are discussed.     

Chaos-Coded Modulations Over Rician and Rayleigh Flat Fading Channels

In this brief, we analyze a kind of chaos-coded modulations over both Rician and Rayleigh frequency non-selective uncorrelated fading in the presence of additive white Gaussian noise. We provide bounds both for the case when perfect channel-state information (CSI) is available at the decoder and when there is no CSI. We show that the bounds proposed can be tight enough to give reason of the behavior of these systems in a flat fading channel. We compare the results with a related trellis-coded modulation and show that the degradation in performance can be at least as low as with conventional coded modulation systems.     

SVD-Based Techniques for Zero-Padded Block Transmission Over Fading Channels

In this paper, the performance of filter bank transceivers in the presence of a dispersive time-variant channel is investigated. It is well known that filter bank transceivers can be adapted to the channel transfer function to yield intersymbol interference (ISI) cancellation. When the channel is time variant, several problems arise, since the transceiver should be changed whenever the channel evolves. In this paper, we will allow both the transmitter and the receiver to change and satisfy the interference-free condition, under the assumption of a zero-padded block transmission. In this case, the optimum transmitter-receiver pair can be computed by using a singular value decomposition (SVD) of the channel matrix. A fast receiver adaptation based on SVD tracking is presented. Simulation results show that minimum performance loss with respect to the optimum receiver can be achieved for our reduced complexity receiver     

Differential Space-Time Modulation Using DAPSK Over Rician Fading Channels

This paper studies differential space-time modulation using diversity-encoded differential amplitude and phase shift keying (DAPSK) for the multiple-input multiple-output (MIMO) system over independent but not identically distributed (inid) time-correlated Rician fading channels. An asymptotic maximum likelihood (AML) receiver is developed for differentially detecting diversity-encoded DAPSK symbol signals by operating on two consecutive received symbol blocks sequentially. Based on Beaulieu’s convergent series, the bit error probability (BEP) upper bound is analyzed for the AML receiver over inid time-correlated Rician fading channels. Particularly, an approximate BEP upper bound of the AML receiver is also derived for inid time-invariant Rayleigh fading channels with large received signal-to-noise power ratios. By virtue of this approximate bound, a design criterion is developed to determine the appropriate diversity encoding coefficients for the proposed DAPSK MIMO system. Numerical and simulation results show that the AML receiver for diversity-encoded DAPSK is nearly optimum when the average received signal-to-noise power ratios are high and the channel is heavily correlated fading and can provide better error performance than conventional noncoherent MIMO systems when the effect of non-ideal transmit power amplification is taken into account.     

Channel Independent Precoder for OFDM-Based Systems Over Fading Channels

In this paper we propose an independent channel precoder for orthogonal frequency division multiplexing (OFDM) systems over fading channels. The design of the precoder is based on the information redistribution of the input modulated symbols among the output precoded symbols. The proposed precoder decreases the variance of the instantaneous noise power at the receiver produced by the channel variability. The employment of an interleaver together with a precoding matrix whose size does not depend on the number of data carriers in an OFDM symbol allows different configurations of time-frequency diversity which can be easily adapted to the channel conditions. The precoder is evaluated with a modified Zero Forcing (ZF) equalizer whose maximum gain is constrained by means of a clipping factor. Thus, the clipping factor limits the noise power transfer in the receiver deprecoding block in low SNR conditions.      

Optimization of Switched MIMO Systems Over Rayleigh Fading Channels

This paper proposes a new switched MIMO system where the MIMO includes both diversity-based MIMO and spatial multiplexing MIMO (SM-MIMO). The proposed scheme is a closed-loop system where the feedback bits are sent back to the transmitter to indicate the channel quality. The switching operation is employed among groups of a certain number of transmit antennas. For diversity-based MIMO systems, the switching operation aims to improve the link performance, while the goal of the switching for the SM-MIMO systems is to increase the data rate. The authors analyze and optimize the proposed systems and show that there is considerable benefit in terms of performance and practical use